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Withdrawal Of Southwest Monsoon, A Gateway To Northeast Monsoon In South India

October 8, 2021 6:53 PM |

Southwest monsoon commenced withdrawal on 06thOctober, retreating from some parts of West Rajasthan and Gujarat, to start with. Further withdrawal is likely from most parts of North India, hills and plains both, in the next 3-4days. Complete withdrawal from the central parts covering entire Madhya Pradesh and Maharashtra will still be a long drawn process and may roll over to 2nd half of October.

Withdrawal of monsoon has more variability than the onset. Complete withdrawal was accomplished in one week in 2019. The most delayed withdrawal ever started on 09thOctober and ended on 17thOctober. Contrarily, in 2020, the retreat started on 28thSeptember, 11days later than the revised normal date of 17thSeptember and the process was complete only after one month, on 28thOctober.

Monsoon withdrawal is dictated by changes in the meteorological features observed at 850mb level (5000’ above mean sea). Monsoon is announced to withdraw from a region when the following criteria is met: An anticyclone establishes at 5000’ above sea level; rainfall activity ceases for 5 continuous days; and moisture content reduces sufficiently as inferred from the satellite water vapour imageries.

The monsoon withdrawal dates got revised in 2019. Prior to this, the normal date of withdrawal from West Rajasthan was taken as 01stSeptember and this was postponed to 17thSeptember, based on data from 1961 to 2019. Notwithstanding the revised markers of withdrawal, still the monsoon has commenced late retreat for the 5thconsecutive year.

The southwest monsoon had seen the most delayed withdrawal in 2019, on 09th October. Before 2019, the most delayed withdrawal was recorded in 1961.  Earlier, in 2005 and 2015, the monsoon commenced withdrawal on 02ndSep and 04thSep respectively, closest to the normal date of 01stSptember.

The  normal date for the monsoon withdrawal from the entire country (15°N) is fixed at 15thOctober. However, this threshold also has been overstepped on many occasion and with a big margin on few of them. Invariably, the late withdrawal becomes consequential for onset of northeast monsoon over Tamil Nadu and South Peninsula.

During last year itself, the southwest monsoon withdrew completely on 28thOctober and the date coincided with the delayed onset of northeast monsoon. Simultaneous existence of both the monsoons is not realistic and therefore is best avoided. The onset of northeast monsoon is not considered before 10th October and the normal date is taken as 20thOctober, subject to fulfilment of set of environmental conditions.

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